In PVD processes and more in particular in continuous or semi-continuous PVD processes on an industrial scale it is necessary to provide a stable supply of liquid metal to the evaporator device wherein the supply is in accordance with the liquid metal evaporated in the evaporator device. This is particularly demanding with high speed PVD processes such as the deposition of evaporated metal on steel strip.
Moreover, with a liquid metal composed of two or more metals with different vapour pressures, the metal or metals with the higher vapour pressure will evaporate at a higher rate than the metal or metals with a lower vapour pressure. As a result of that the composition of the liquid metal in the evaporator will change over time. Since the metals used in the composition of the liquid metal will almost never be 100% pure this will result in that the composition will be enriched with impurities with a lower vapour pressure.
Both the change in composition of the melt and the enrichment of the melt with impurities will have a negative result on the composition of the vapour and hence on the coating deposited on the substrate, as well as on the evaporation process as such.
The problem of impurities could possibly be addressed by using high purity feeding material but this will increase costs considerably and is not really an option if the PVD process is run on an industrial scale.
The change of the composition of the liquid metal could be controlled to a certain extent by increasing the size of the evaporator but this will increase the space needed for the evaporator in the vacuum chamber and eliminates one of the advantages of using induction heating which is having a high power input into a limited space. An alternative is to feed the evaporator from an external source of liquid metal as is disclosed in KR20110034420, which however suffers from the same problem only it will take some more time for the composition to change to an unacceptable level of deviation from the required composition.